The present invention relates to a DC-to-DC converter circuit and in particular to a synchronous flyback converter circuit for operation in a continuous mode.
In DC-to-DC power supplies for different kinds of electrical devices, power rectifiers are utilised in order to obtain a rectified output voltage. Typically, a diode would be employed on the secondary side in order to obtain the rectified output voltage.
A DC-to-DC converter is described in U.S. Pat. No. 5,886,881 for Xia et. al. The forward DC-to-DC converter described can be used to carry out synchronous rectification and zero voltage switching. This circuit is only intended for forward and forward derived converters and is not applicable to flyback converters.
Another DC-to-DC converter is described in U.S. Pat. No. 5,726,869 for Yamashita et. al A synchronous rectifier type forward DC-to-DC converter is disclosed that is capable of preventing an increase in losses occurring when a synchronous rectification switch and a flywheel synchronous rectification switch is conducting simultaneously, or when the synchronous rectification switch is conducting simultaneously as a primary side switch.
If these secondary side switches, which are responsible for rectification on the secondary side of the transformer, are turned on simultaneously, a risk exists of destroying FETs or windings due to large currents. The cross-conduction of the switches is here prevented by physical components, such as the saturable cores, causing some increase of power losses and an increased physical size of the converter.
Another way to design a suitable rectifier circuit is to use flyback topology. In a converter using flyback topology, a primary side stores magnetic energy in an air gap of a magnetic core, or the like, during a charging interval. The energy is then fed to a secondary side during the so called flyback interval. The main advantage of a power converter circuit employing a flyback topology compared to other converter circuits is the simple construction thereof, which makes the cost of manufacturing it low.
A conventional flyback converter comprises, on the primary side, a primary winding of a transformer and a switch, and on the secondary side, a secondary winding of the transformer connected to a diode and an output capacitor over which a load can be connected.
Such a converter has a large voltage drop over the diode. When the output voltage over the output capacitor is low, the voltage drop over the diode becomes a significant part of the overall voltage, which makes the power converter inefficient for such low voltage applications.
The continuously increasing demand to minimise the size of flyback converters, regardless of power class, intensifies the demand on the efficiency of the converters. The efficiency sets the limit of the power that can be converted in a small space in which heating must be kept at a permissible level. The voltage drop in a Schottky type diode is about 0.3 V, or higher. This voltage drop causes one of the greatest losses in a converter. If the voltage drop could be minimised by using a component having a much smaller voltage drop, the power converter would have a significantly higher efficiency. For instance, a MOSFET would reduce this voltage drop.
In the Swedish patent application No. 9804454-8 a continuous mode flyback converter having a synchronous switch is described. The control signal for the drive circuit of the secondary side synchronous switch is taken directly from a secondary or an auxiliary winding of the flyback transformer. The drive pulse goes to a secondary switch on the secondary side, which is generated by an inverting buffer circuit that is fed from the output voltage terminal. The pulse generating circuit generates a drive signal to the synchronous switch, which is independent of the input voltage. One disadvantage of this circuit is that in some conditions the timing of the secondary switch may be inappropriate.
It is an object of the invention to overcome the problems indicated above.
It is another object of the invention to increase the efficiency of electrical DC converters in a flyback topology.
The objects are obtained by a circuit and a method for making synchronous rectification in a flyback converter. The converter is independent of the input voltage on the primary side, and there is no need for an auxiliary winding on the transformer.
Definition of terms used herein.
Flyback transformer: Either a choke, an auto-transformer or a full transformer. The primary winding of a flyback transformer refers to the winding through which the current flows during the forward phase of the converter, and the secondary winding refers to winding through which the current flows during the flyback phase.
Flyback phase: In a flyback converter, the flyback phase is when the primary side switch or main switch is an off-state, i.e. non-conducting, and the secondary side switch, rectifier switch, is an on-state, i.e. conducting. In a special case called a discontinuous mode, the secondary side switch may be turned off when the current through the switch equals zero.
Forward phase: In a flyback converter, the forward phase when the primary side switch is in the on-state, and the secondary side switch is in the off-state.
Discontinuous mode: A mode in a flyback converter, in which the magnetsing energy of the flyback transformer during some time of a switch cycle is equal to zero.
Continuous mode: A mode in a flyback converter, in which the magnetising energy of the flyback transformer during a switch cycle never goes to zero.
On-state: a conducting state for a switch.
Off-state: a non-conducting state for a switch.
Switch: An element having two distinct states, an on-state and an off-state.
Charged controlled circuit: A device which depends on charging of a control terminal, e.g. for a MOSFET, the gate terminal. Examples of charged controlled devices are MOSFETs and IGBTs.
The flyback converter has as conventional a primary side comprising a control device and a first element acting as a main switch. The converter also has a secondary side, comprising a drive circuit and a second element acting as a rectifying switch. The drive circuit has a turn-on part comprising a first transistor, a first diode between a base an emitter of the first transistor and a drive capacitor, which is connected to the control device. The drive circuit also has a turn-off part, comprising a second transistor, a second diode between a base and an emitter of the second transistor, and a second drive capacitor, which is connected to the control device.
Drive pulses from the control device at the primary control the flyback converter and are issued as follows:
At a turn-off signal. e.g. a negative flank of a drive pulse, the main switch is turned off. The turn-off signal also activates the turn-on part of the drive circuit so that the rectifying switch conducts.
At a turn-on signal, e.g. a positive flank of a drive pulse, the main switch is turned on. The turn-on signal also activates the turn-off part of the drive circuit so that the rectifying switch becomes non-conducting.
One advantage of the flyback converter as described herein is that it comprises few components.
Another advantage of the flyback converter is that the first and the second transistors on the secondary side cannot conduct simultaneously, although they are triggered by pulses from the same control device. This results from the fact that the first transistor is in an on-state only during the turn-off signals of the control signal, and that the turn-off transistor is in an on-state only during the turn-on signals. The rest of the time both the first and second transistors are in off-states.
Yet another advantage is that the voltage feed to the rectifying switch and the drive circuit does not vary with the input voltage or the output current of the flyback converter.
Another advantage is that the flyback converter prevents the main switch from conducting simultaneously with the rectifying switch.
A further advantage is the simple way of applying appropriate delays at the turn-on signal and in particular at the turnoff signal of the rectifying switch.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.